The solubility of a protein depends on the net charge on the surface of the protein molecule. The net charge depends on the pH of the solvent and the number and identities of the amino acids that make up the protein. The isoelectric point of a protein occurs at a specific pH when the positive and negative charges balance each other out and the net charge is zero. At this isoelectric point a protein is least soluble. For most proteins this occurs in the pH range of 5.5 to 8. A protein gets more soluble if there is a net charge at the protein surface, since it prefers to interact with water, rather than with other protein molecules. Without a charge at the surface proteins are likely precipitate and aggregate.